Silver is a well-known agent for imparting a yellow or brown color to glasses. The coloring effect of silver in glass is attributed to the absorption behavior of metallic silver aggregates of colloidal size. These may be precipitated by heat treatment of glasses containing dissolved silver, or introduced into the glass surface by ion-exchange processes at elevated temperatures utilizing so-called silver staining techniques. A comprehensive discussion of the use of silver as a glass coloring agent is provided by W. A. Weyl in Coloured Glasses, Dawson's of Pall Mall, London (1959).
Methods for utilizing silver to provide colors other than yellow or brown in certain glasses are also known. Thus Forst et al. describe, in "Red Silver Glasses", J. Am. Cer. Soc., 25, 10, pages 278-280 (1942), the use of silver to produce yellow, orange, red and purple colors in certain lead silicate glasses containing an immiscible halide phase. The disadvantages of this system include the need for large additions of silver halides, the tendency toward opalization on reheating exhibited by some of the glasses, and the fact that only a limited range of colors (no blue or green) may by produced therein.
The phase-separation behavior which occurs in the alkali borosilicate glass composition system is documented by Hood et al. in U.S. Pat. No. 2,106,744. Hood et al. found that many such glasses separate during forming or upon reheating into a major silica-rich phase and a minor boron-rich phase.
The distinct nature of the boron-rich phase is shown by the fact that it can be removed from the silica-rich phase in certain of these glasses by suitable chemical treatments. The primary application for these phase-separable borosilicates has been in the manufacture of highly refractory glass products known as 96% silica glasses.